1. Field of the Invention
The present invention relates to a fluid coupling, and particularly to a fluid coupling without an inner core.
2. Description of the Related Art
A fluid coupling (hereinafter "coupling") operates to transmit power through fluid between a pump impeller and a turbine runner which are opposed to each other. A coupling does not function to increase torque, and thereby differs from a torque converter, but, rather, functions simply as a coupling for transmitting power. Since fluid couplings do not have a stator, they are smaller and light in construction, and thus have been used as starting devices in vehicles.
Couplings are classified in accordance with whether or not they have an inner core for guiding the flow of fluid in the coupling. The "core type" has such an inner core and the "coreless type" lacks the inner core. An "inner core" is shown, for example, as member 8 in U.S. Pat. No. 5,005,356 issued to Saunders and as elements 10b in U.S. Pat. No. 4,866,935 issued to Hayabuchi et al.
Since the flow in the coupling of the coreless type has not been sufficiently analyzed, it is difficult to predict performance when designing a coupling. Further, the fluid in a coupling of the coreless type is rapidly compressed and decompressed when the blades of the impeller and turbine pass each other, which may cause cavitation resulting in vibration and blade noise. Couplings provided with cores have been used in some industrial vehicles, ships, industrial machines and other applications in noisy environments where the noise generated by operation of the coupling is not deemed a problem. However, couplings of the coreless type have not been put to practical use for purposes such as automobiles in which silence is required. Conversely, the performance of the couplings of the core type can be estimated relatively easily. Also, the provision of a core increases the rigidity, and thus the vibration of the blades and the resulting noise can be prevented.
Passenger cars tend to be designed to give the feeling of a high-class car. Automobiles desirably emulate a car equipped with a torque converter and automatic transmission, i.e. , a sensation of rapid acceleration responsive to depression of the accelerator pedal and a feeling of coasting when the accelerator pedal is released. With a torque converter, the above sensations can be realized owing to the provision of the stator between the pump impeller and the turbine runner, which stator enables the turbine to have a configuration which optimizes reception of fluid discharged from the pump during forward driving, while minimizing transfer of the fluid from the turbine in reverse driving. Thus, a torque converter generally has a small capacity coefficient when operating in reverse.
Another tendency in automobile design is toward sportiness. Since "sporty" automobiles require rapid response characteristics, it is desirable to have a feeling of acceleration which reflects the engine characteristics during positive acceleration. It is also desirable to provide a strong feeling of engine braking in negative acceleration, i.e., during engine braking in the off-state of the accelerator.
Various gear ratios are provided in automobiles in order to provide a good sensation of acceleration and engine braking. However, in order to comply with the demand for the abovedescribed driving "feel" it is essential to be able to arbitrarily set the capacity coefficient in a coupling.
In known couplings the blades extend radially, i.e., along normal lines, from a center of rotation, and the performance is adjusted by inclining the blades with respect to the rotational axis of the pump and turbine at an appropriate angle .theta..sub.1. If the blade angle .theta..sub.1 is adjusted to increase the capacity coefficient in forward driving, the capacity coefficient in reverse driving is also increased, so that it is difficult to obtain the desired coasting feeling mentioned above. Thus, if a coupling without a core is designed to increase capacity coefficient in forward driving, a like increase of the capacity coefficient in reverse driving operation can be avoided only to a slight extent, but cannot be eliminated to an extent enabling the desired coasting feeling.
Approaches to solving the above problems have been disclosed in the Japanese Laid-Open Patent Publication Nos. 2-159425 (159425/1990) and 1-208238 (208238/1989). The structure disclosed in the former includes a baffle plate for baffling the flow of fluid so as to suppress the capacity coefficient in reverse. The structure described in the latter publication is designed to reduce the capacity coefficient by reducing the pressure within the coupling.
However, the former technique is applied only to couplings having inner cores for fluid guidance in a coupling provided with a core, the fluid flows between the pump and impeller through an inlet and outlet which are located at constant positions. Therefore, it would be pointless to apply this approach to a coupling without a core, in which the positions of the inlet and outlet are not constant. Further, the baffle plate in the coupling intentionally causes turbulent flow, so that the transmission efficiency decreases, and thus the fuel consumption is lowered. The characteristics in forward driving are also adversely affected. Moreover, the number of parts is increased, resulting in increase of the weight and cost. Additionally, fixing the baffle plate is difficult, and the reliability of the product is compromised.
On the other hand, the latter prior art approach in which the internal pressure is adjusted, requires complicated control, and also requires a dedicated circuit for reducing the internal pressure, resulting in an increase in the number of parts and thus an increase in cost.
In view of the foregoing, it is an object of the invention to provide a coupling, in which an inner core is eliminated to reduce the number of parts and improve the performance, while capacity coefficients in the forward and reverse driving can be freely set so as to comply with various demands depending on types and specifications.